A high-precision opto-mechanical breadboard for a lens mount has been assembled by means of a laserbased
soldering process called Solderjet Bumping; which thanks to its localized and minimized input
of thermal energy, is well suited for the joining of optical components made of fragile and brittle
materials such as glasses. An optical element made of a silica lens and a titanium barrel has been studied
to replicate the lens mounts of the afocal beam expander used in the LIDAR instrument (ATLID) of the
ESA EarthCare Mission, whose aim is to monitor molecular and particle-based back-scattering in order
to analyze atmosphere composition. Finally, a beam expander optical element breadboard with a silica
lens and a titanium barrel was assembled using the Solderjet Bumping technology with
Sn96.5Ag3Cu0.5 SAC305 alloy resulting in a low residual stress (<1 MPa) on the joining areas, a low
light-depolarization (<0.2 %) and low distortion (wave-front error measurement < 5 nm rms) on the
assemblies. The devices also successfully passed humidity, thermal-vacuum, vibration, and shock tests
with conditions similar to the ones expected for the ESA EarthCare mission and without altering their
Solder joining is an all inorganic, adhesive free bonding technique for optical components and support structures of advanced optical systems. We established laser-based Solderjet Bumping for mounting and joining of elements with highest accuracies and stability. It has been proven for optical assemblies operating under harsh environmental conditions, high energetic or ionizing radiation, and for vacuum operation. Spaceborne instrumentation experiencing such conditions and can benefit from inorganic joining to avoid adhesives and optical cements. The metallization of components, necessary to provide solder wetting, mainly relies on well-adhering layer systems provided by physical vapor deposition (PVD). We present the investigation of electroless Ni(P)/Pd/Au plating as a cost-efficient alternative under bump metallization of complex or large components unsuitable for commercially available PVD. The electroless Ni(P)/Pd/Au plating is characterized with respect to layer adherence, solderability, and bond strength using SnAg3Cu0.5 lead-free solder alloy.
Advanced optical systems of telescopes and scientific instrumentation require high accuracy mounting and joining of components. Applications for deep UV, under high energetic radiation, for vacuum operation, or assemblies subjected to environmental loads (e.g. humidity and temperature) require a replacement of organic adhesives or optical cement by a more robust bonding agent. Soldering allows the bonding of different materials with an inorganic filler material. We present the optimization of the laser-based Solderjet Bumping for the mounting of optical components and the parameters of the bonding process for fused silica and LAK9G15 (radiation resistant glass) with thermally matched metal mounts. The investigation covers the experimental determination and optimization of solder wetting to the respective base materials and the bond strengths achieved.
Solder joining using metallic solder alloys is an alternative to adhesive bonding. Laser-based soldering processes are especially well suited for the joining of optical components made of fragile and brittle materials such as glasses, ceramics and optical crystals. This is due to a localized and minimized input of thermal energy. Solderjet bumping technology has been used to assemble a lens mount breadboard taking as input specifications the requirements found for the optical beam expander for the European Space Agency (ESA) EarthCare Mission. The silica lens and a titanium barrel have been designed and assembled with this technology in order to withstand the stringent mission demands; handling high mechanical and thermal loads without losing its optical performances. Finally a high-precision opto-mechanical lens mount has been assembled with a minimal localized stress (<1 MPa) showing outstanding performances in terms of wave-front error measurements and beam depolarization ratio before and after environmental tests.
A novel laser-based soldering technique – Solderjet Bumping – using liquid solder droplets in a flux-free process with only localized heating is presented. We demonstrate an all inorganic, adhesive free bonding of optical components and support structures suitable for optical assemblies and instruments under harsh environmental conditions. Low strain bonding suitable for a following high-precision adjustment turning process is presented, addressing components and subsystems for objectives for high power and short wavelengths. The discussed case study shows large aperture transmissive optics (diameter approx. 74 mm and 50 mm) made of fused silica and LAK9G15, a radiation resistant glass, bonded to thermally matched metallic mounts. The process chain of Solderjet Bumping – cleaning, solderable metallization, handling, bonding and inspection – is discussed. This multi-material approach requires numerical modelling for dimensioning according to thermal and mechanical loads. The findings of numerical modelling, process parametrization and environmental testing (thermal and vibrational loads) are presented. Stress and strain introduced into optical components as well as deformation of optical surfaces can significantly deteriorate the wave front of passing light and therefore reduce system performance significantly. The optical performance with respect to stress/strain and surface deformation during bonding and environmental testing were evaluated using noncontact and nondestructive optical techniques: polarimetry and interferometry, respectively. Stress induced surface deformation of less than 100 nm and changes in optical path difference below 5 nm were achieved. Bond strengths of about 55 MPa are reported using tin-silver-copper soft solder alloy.
Solder-joining using metallic solder alloys is an alternative to adhesive bonding. Laser-based soldering processes are especially well suited for the joining of optical components made of fragile and brittle materials such as glasses, ceramics and optical crystals due to a localized and minimized input of thermal energy. The Solderjet Bumping technique is used to assemble a miniaturized laser resonator in order to obtain higher robustness, wider thermal conductivity performance, higher vacuum and radiation compatibility, and better heat and long term stability compared with identical glued devices. The resulting assembled compact and robust green diode-pumped solid-state laser is part of the future Raman Laser Spectrometer designed for the Exomars European Space Agency (ESA) space mission 2018.
Miniaturization of photonic devices is required by various applications such as data storage and processing, optical
communications, and metrology. This request can be met by new optical designs, miniaturized components, and
advanced packaging technologies. Design, assembly, and characterization of a miniaturized photonic wavelength-division
multiplexing (WDM) device for optical measurements are presented. The device features the use of gradient
index lenses (GRIN-lens) and the utilization of an adhesive free, laser-based joining technology. Solderjet Bumping
offers flux-free soldering in a localized inert nitrogen atmosphere with minimized input of thermal energy, thus allowing
for the joining of fragile materials such as glass or brittle ceramics. The proposed system design consists of a system
platform made of borofloat BF33 with a footprint of approx. 30x20 mm<sup>2</sup>. Mechanical stops also made of borofloat glass,
fiber-ferrules with a length of approx. 5 mm, and GRIN-lenses with a length of 4.05 mm are attached to the base-plate by
solder joints. The solder process uses tin-silver-copper (Sn3Ag0.5Cu) solder spheres with a diameter of 200, 400, and
760 μm. A fiber-to-fiber coupling efficiency of 72 % is demonstrated using uncoated components.
Miniaturized video endoscopes with an imager located at the distal end and a simplified opto-mechanical layout are
presented. They are based on a CMOS imager with 650 x 650 pixels of 2.8 μm pitch and provide straight view with 75° and 110° field of view at f/4.3. They have an outer diameter of 3 mm including the shell and a length of approx. 8 mm.
The optics consist of polymer lenses in combination with a GRIN and a dispensed lens. Using a simple flip chip
assembly, optical axis alignment better than 10 μm and a contrast of 30 % at 90 LP/mm was achieved. The 75° FOV
system was sealed at the front window using a solderjetting technology, providing 10<sup>-9</sup> mbar*l/s leakage rates even after
several autoclave cycles.
Laser beam soldering is a packaging technology alternative to polymeric adhesive bonding in terms of stability and
functionality. Nevertheless, when packaging especially micro optical and MOEMS systems this technology has to fulfil
stringent requirements for accuracy in the micron and submicron range. Investigating the assembly of several laser
optical systems it has been shown that micron accuracy and submicron reproducibility can be reached when using
design-of-experiment optimized solder processes that are based on applying liquid solder drops ("Solder Bumping") onto
wettable metalized joining surfaces of optical components. The soldered assemblies were subject to thermal cycles and
vibration/ shock test also.
This paper reports on new results of the development of a unimorph laser beam shaping mirror based on Low
Temperature Cofired Ceramics (LTCC). The deformable mirror is actuated by a side screen printed piezoceramic thick
film based on lead zirconate titanate (PZT). The reflective surface is electroplated copper that is diamond machined to
flatten the surface. We introduce the solder jet bumbing fixation technology to mount the deformable mirror into a
metallic mounting. This assembling technology introduces very little energy input and thus also very little deformation
into the mirror. The material of the mounting is CE7 that is especially thermal adapted to the deformable mirror. We will
present results on deflection and resonance frequency for two different mirror designs.
Laser based solder bumping is a highly flexible and fast approach for flux-free soldering of micro-optical components in
complex 3D geometries with localized and time restricted energy input. Solder joints provide superior mechanical
strength, higher radiation stability, humidity resistance and a good thermal and electrical conductivity compared to
adhesive bonding. Due to the good long term stability solder joints are feasible for the integration of optical, mechanical,
electronic, and MEMS/MOEMS devices in multi functional hybrid optical assemblies. Comparative studies of solder
bumping of optical components with sputtered thin film metallization on platforms made of Alumina (Al<sub>2</sub>O<sub>3</sub>) and Low
Temperature Cofired Ceramics (LTCC) with both Au and AgPd thick film metallization were carried out using design of
experiment methods (DoE). The influence of the system parameters, laser pulse energy and duration, distance, incidence
angle and nitrogen pressure on targeting accuracy and bond strength were evaluated. The jetting of liquid solder spheres
within a localized nitrogen atmosphere improves wetting on the respective wetting surfaces and simplifies the joining
process due to integration of solder alloy preform handling and reflowing, thus showing great potential for a high degree
Miniaturized optical systems that couple light from broad area or trapezoid diode laser bars with cw powers up to 100 W
into fibers with core diameters between 50 μm and 100 μm have been developed and assembled on smart Direct-Copper-Bond (DCB) system platforms that incorporate active cooling structures as well as hermetic housing facilities. The
approach for a fast and flexible joining of the optical elements by a flux-free applied solder is to jet liquid solder spheres
onto joining geometries, thus enabling for creating complex shaped solder joint geometries with high accuracies.
Laser based joining technologies for optical assemblies overcome the limitations of standard fixation technologies such
as adhesive or wafer level bonding. By applying the laser energy locally and for a limited time these technologies enable
for higher stability of optical joints as well as additional functionality. Working without intermediate layers laser splicing
creates highly stable transparent joints that are suitable for the transmission of high power, e.g. in fiber laser assemblies.
In contrast, laser beam soldering of optical components as an alternative with a metallic intermediate layer is non-transparent,
but creates flexible and stress-compensating joints as well as thermal and electrical interconnects.